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It is completely unclear what you are even asking. I feel like you're trying to disguise asking for support for an old version just by throwing 1.15.2 in the title of your thread. Versions older than 1.14 are not supported on these forums, and as you've mentioned, thread will get locked with support requests for these old versions. Currently: Items are registered either using registry events (old way) or using DeferredRegister. The Javadocs and Cadiboo's example mod, or even searching these forums for "DeferredRegister" will find you examples. Recipes are added via json (except potion recipes, which is a whole other thing). There are a million examples of json recipes in the vanilla files, and the minecraft wiki has a good breakdown of the syntax and available elements. Please attempt what you are wishing to achieve, and if it does not work, post code (or a link to a github, which is better), logs, and exactly what you are trying to achieve from the end-user standpoint.

Forge still has to do work even without any mods installed. Arguments about efficiency are rarely fruitful, a few extra seconds of loading for hours of extra content shouldn't be a dealbreaker.